1. Field of the Invention
This invention relates to a method of manufacturing a spherical body comprising a core and at least one surface layer enclosing the core. The invention relates also to spherical bodies manufactured by this method, and especially two-piece and multi-piece golf balls.
2. Prior Art
Two-piece golf balls, which have become very popular in recent years, are manufactured by placing a special rubber core as the nest within a two-part mold having upper and lower halves, each with a hemispherical concave inner face, holding the core at the center with pins provided for retraction near the bottom and top of the inner face of the mold, and injection molding a thermoplastic resin such as ionomer resin in the gap between the core and the inner face of the mold.
The manufacture of multi-piece golf balls having two or more surface layers requires the same number of surface layer-forming molds, such as a mold for the inner surface layer and another for the outer surface layer, as there are surface layers. The injection molding process described above is carried out a plurality of times with different molding materials.
However, efforts to form very thin surface layers having a thickness of 1.5 mm or less in the gap between the core serving as the nest and the inner face of the mold using a prior-art injection molding process have met with difficulties. The thermoplastic resin material which has been forced through a runner from the injection molding machine cools and prematurely cures as it is spreads throughout the entire region of the gap, making it likely that unduly thin-walled sections and unfilled areas will arise.
Moreover, the molding of multi-piece balls, which have a multilayer construction, requires that a plurality of molds of differing inner face diameters be furnished and that injection molding be carried out a plurality of times, once for each surface layer. This in turn increases the equipment and labor involved in producing the balls.
Accordingly, one object of the present invention is to provide a spherical body having at least one very thin surface layer of uniform thickness. Another object of this invention is to provide a method for the manufacture of such spherical bodies which has an excellent productivity efficiency.
The invention achieves these objects by providing a method of manufacturing a spherical body comprising a core and at least one surface layer enclosing the core, the method comprising preforming, surface layer forming and molding steps. In the preforming step which is carried out in a two-part mold having two mold halves, each with a hemispherical concave inner face defining a cavity, a pair of sheet blanks each having opposed sides are placed over the cavities in the two halves of the mold such that one side of each blank faces the inner face of one mold half, then the opposite side of each blank is pressed while vacuum suction is applied to the one side facing the mold inner face, for drawing each sheet blank up against the inner face of the respective mold half, thereby forming a pair of half-shells. In the surface layer forming step which is carried out in another two-part mold having two mold halves, each with an inner face bearing pattern-forming projections, the mold halves are separated, the pair of half-shells having inner and outer sides are loaded into the respective halves of the mold such that the outer side of each half-shell faces the inner face of the corresponding mold half, the core is placed on one of the mold halves, and the other mold half is joined thereto to close the mold, thereby pressing the core against the inner sides of the half-shells, concurrent with which vacuum suction is applied to the outer sides of the half-shells from the respective inner faces of the mold halves. In the molding step, the surface layer is fused and solidified onto the core within the closed mold.
The invention also provides a spherical body which has been manufactured by the inventive method. Any one of the surface layers has a thickness of not more than 1.5 mm.